1. Technical Field
This invention relates to reflective telescope systems and, more particularly, to an all-reflective multi-focal length, multi-field of view three-mirror anastigmat.
2. Discussion
The performance of a conventional high quality telescope when used on the earth for celestial viewing is principally limited to the earth's atmosphere rather than by the construction of the telescope. Atmospheric effects not only limit the resolution of telescopes, but also absorb large portions of the electromagnetic spectral range. While in the past, very little could be done about this limitation, today, with the help of earth satellites and other space vehicles, it is possible to place a telescope above the atmosphere and perform extraterrestrial observations without interference from it. As a result, there has arisen a need for telescope which can more fully take advantage of this new environment. Also, it should be understood that the present inVention may be utilized for other airborne or ground based applications that require viewing distant objects.
One such telescope is that disclosed in U.S. Pat. No. 4,101,195, issued to Korsch. This reference discloses an anastigmatic three-mirror telescope. The telescope has a primary and secondary configuration similar to a Cassegrain design which forms a real image closely behind the primary mirror. The image is then relayed or reimaged by a tertiary mirror at approximately unit magnification to a flat plane for viewing, the plane being parallel to or displaced from the axis of the primary and secondary mirrors. To achieve this, a plane mirror, which functions as a fold mirror, is positioned at an angle of 45.degree. with respect to the image formed by the primary-secondary mirrors and either directs light from the secondary mirror to the tertiary mirror or reflects light from the tertiary mirror to a final image plane.